Large fans having diameters ranging from about one to ten meters or more are commonly used for moving air through cooling towers, heat exchangers and the like. A typical fan in such an application may have a diameter of about five meters and anywhere from two to eighteen airfoil shaped blades coupled to a rotatable hub. For light weight and economy, such fan blades may be fabricated from thin aluminum alloy sheets. The sheet metal is bent to provide a rounded leading edge, with the upper and lower surfaces of the blade converging toward a trailing edge where they are riveted or spot welded together. The chord line of the airfoil blade at the tip of the blade ranges anywhere from about fifteen to forty centimeters, and the maximum thickness of the airfoil ranges anywhere from about two to fifteen centimeters.
As used herein, the downstream face of the fan and blades is referred to as the upper face and the upstream face is referred to as the lower or cambered face. This is because the largest of the fans are primarily used in cooling towers or the like where they rotate about a vertical axis. Such fans are also used where the fan rotates around a horizontal axis.
Such large air-moving fans operate within a circumferentially extending shroud, which is very often not quite circular and may not be exactly concentric with the axis of the hub. Therefore, when a fan is installed, the blades and/or shroud are adjusted so that the blades clear the inside of the shroud by one or two millimeters at the closest approach, however, the blades may be about twenty millimeters (or greater) away from the shroud at the widest gap.
The blades of large fans of The Moore Company of Marceline, Mo., the assignee of the present application, are mounted to a central hub, preferably by a connection that permits limited vertical (parallel to the axis of rotation) motion. Thus, the blades may "droop" slightly when stopped, but generally extend radially from the hub during rotation. The connection between the inner ends of the blades and the hub is critical since it is a possible source for failure by fatigue cracking. Light weight and reliability are important. It is desirable to provide a mounting for blades which has minimum susceptibility to fatigue failures.
A clevis is typically used in these fans to mount the airfoil blades onto the rotatable hub. Although these fans are quite operational, there is always a need to improve the overall performance of large air-moving fans. Since one of the components of the fans that affects overall performance is the mounting of the blades, there is always a need for an improved means of fan blade mounting.